A data backup system using two calculating systems has been used widely. The term “data backup” means preparing the same data as that of one calculating system, and storing it in the other calculating system. The calculating system of a backup source is referred to as a primary system, and the calculating system of a backup destination is referred to as a secondary system. Further, the data stored in the secondary system is referred to as a backup data. The data backup by a duplex system is disclosed in, for example, CHRISTOS A. POLYZOIS, HECTOR GARCIA-MOLINA, “Evaluation of Remote Backup Algorithms for Transaction-Processing Systems”, ACM Transactions on Database Systems, Vol. 19, No. 3, September 1994, Pages 423-449 (Non-Patent Document 1).
As a backup system by the duplex system, a log transfer system has been known. This log transfer system is a system where a data update log of the primary system is transferred to the secondary system and backup data is prepared in the secondary system. In this log transfer system, at the stage of starting the backup, the data of the primary system and the backup data of the secondary system are made identical. After starting the backup, the data update log of the primary system is transferred to the secondary system. In the secondary system, based on the transferred data update log, the backup data is updated to prepare a latest backup data. The process for reflecting the data update log on the backup data is referred to as a log application. The log transfer system is disclosed in the above-described Non-Patent Document 1.
In the log transfer system, in order to confirm that the backup data is normally prepared, it is necessary to verify the consistency of the data of both systems. As a method for verifying the consistency of two data, a checksum method in which the data is regarded as a numerical value and a total of such values is checked is known. When verifying consistency of the data of both systems, a checksum of the data of the primary system and a checksum of the backup data of the secondary system are checked, thereby verifying whether or not the data are consistent.
As software for managing the data of the calculating system, a Database Management System (hereinafter, referred to as DBMS) has been in wide use. In general, the DBMS manages data by using a data main body and a data update log, and the above-described log transfer system has been used widely for the backup of the data managed by the DBMS.
In general, the DBMS manages the data main body by splitting it into data areas of a given size (for example, 8 KB). This data area is referred to as a page, and is managed with a number attached on it. In the data update log, page numbers and contents of data update are written. For example, when data having an Item_ID of 1 and a Quantity of 10 is added onto a page of the page number 100, a data update log with the content of [data having an Item_ID of 1 and a Quantity of 10 is added onto the page number 100] is recorded. In the secondary system having received such data update log, the data having “an Item_ID of 1 and a Quantity of 10” is added onto the page number 100 of the backup data.